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No* 523 

By Ludwig Kuchel 

From Schweissen, Schneiden und 
Metallspritzen mittels Acetylen, 1927 


P.O. BOX 117011 ^^^^_^,.jQA 
6AlNESVILte.FL 32611-7011 U3^ 



July, 1929 

Ifif9>'l 9'^^ ^s-iH^^o 



By Ludwig Kuchel. 

In the original manufacture of airplanes from vrood, the 
warping of the structural parts through the influence of the 
elements could not be entirely prevented, despite the careful 
selection of the wood. An improvement was made by using plywood 
for covering the fuseletge and icings. Plywood was used in Ger- 
many during the war on most of the military airplanes. 

The increased requirements of airplanes, especially irr mod- 
ern air traffic, caviled for a homogeneous building materiaJ. of 
greater strength and reliability than wood, which is so easily 
affected by external conditions. 

The advajitages of steel for highly stressed parts, a.s dis- 
covered in engine building, led, in airplajae construction, to 
the use of steel tubing, which also offers less obstacles to 
future structural development. Autogenous acetylene-oxygen 
welding was found to be the only practical way to join the steel 

Lightness is an essentia2 chejact eristic of the structuraJL 
parts of an airplane. All the structural requirements which 
must be satisfied by the building materials must therefore be 

adequate without being excessive. While militajcy airplanes are 

*"Die autogene Schweissung im Flugzeugbau, " from Schweissen, 
Schneiden und Met all sprit zen mittels Acetylen, 1927, pp. 27-33, 

N.A.C.A. Technical Memorcmdum No. 523 2 

not subject to considerations of economy, "but only to those of 
maximum efficiency, comi'iiercial airplajies must be governed by 
both of these factors. 

The structural parts of an airplane are the fuselage, wings, 
tail surfaces, engine mount and landing gear. 

Figure 1 is the picture of a tvdn-engine Albatros coitimer- 
cial airplane. It has a full load of 3800 kg (8378 lb.) and aji 
engine power of 480 HP. 

Fuselage .- The steel-tube framework of a twin-engine Alba- 
tros commercial airplane (Fig. 2) for eight passengers and two 
pilots has a framework of approximately square cross section 
in the form of a lattice girder. The front part has K braces. 
The rear fields are crossed by brace wires with the exception: 
of the next to the last field, in which there are welded diago- 
nal tubes for withstanding the stresses produced by the tail 
skid in landing. The fuselage framework weighs 224 kg (494 lb»)* 
The joints are autogenously welded in all the structural parts. 
The longerons consist of telescoped steel tubes of 40 mm (1*57 
in.) diameter at the front end and tapering to 25 mm (0.98 in») 
at the rear end. The ends of the tubes joined by sloping 
welds. The requisite reinforcement is effected by muffs, which 
are driven on and welded. The attachment joints of the wings 
and landing gear, which are sub.ject to great tensile or com- 
pressive stresses, are reinforced by strips of sheet metal driven 

IJ.A.C.A. Technical Merriorrjid-ura No. 523 3 

into slots in the ends of the tubes and completely welded. The 
eyelets required for attaching the bra.ce wires are formed by 
welded-in: tubular loops. , No tube in the whole fuselage has a 
wall thicker than 1 mm (0.04 in.), but the required safety fac- 
tor of eight for the airplane is nevertheless exceeded. 

Wings. - Figure 3 shows the whole wing structure, consist- 
ing of a middle section: and two lateral sections. Figure 4 
shom^s a wing in process of construction. The spars are full- 
walled box girders with open duralumin: members. The fitted-im 
steel ribs have the form. of lattice girders. The open spaces 
in the front part of the wings are intended for the reception: 
of the fuel tanks. 

The welded steel-tubing engine supports, which also serve 
as wing struts, are installed between the upper and lower wings 
on each side of the fuselage. 

Figure 5 shows two of the ribs. The upper and lower 
flanges are made of steel tubing 6 x 0.5 ram (0,24 x 0.02 in.), 
while the welded-in lattices are made of steel tubing 5 x 0.5 mm 
(0.2 x 0.02 in.). A spaji of 19 m (82.33 ft.) requires 65 such 
ribs for each wing. These ribs are produced in qua.ntity and 
are welded with the 3,id of formers. 

Figure 6 shows the tail structures. The horizontal tail 
structures, consisting of the stabilizer and elevator, have 
welded frames with diagonal braces made from steel tubing of 

N.A.CA. Technical Memorandum ITo. 523 4 

5 to 25 mm (0.2 to 1 in,). The stabilizer is adjustaole, and 
the elevator is hinged to it. The vertical tail structures, 
consisting of the fin-j and rudder, are likewise made of autoge- 
nously welded steel tubing. 

The ailerons are likewise made of welded steel tubing with 
diagonal bracing. The hinge supports are made of steel tubing 
with triangular bracing and are autogenously welded to the T/ing 

Power plaixt ,«. Figure 8 shows the engine mount on a one- 
engine commercial airplane. The autogenously welded steel-r 
tubing engine mount is attached to the steel-tubing fuselac;e 
at four points, so that the whole can be detached by removing 
four bolts. The joints are reinforced by sheet metal, because 
of the great stresses to which they are subjected. The weight 
of the 220 HP. engine, with all its accessories, is 295 kg 
(650 lb.). It is separated from, the pilot room by a fire wall. 

Landing ge^x .- The streamlined struts are autogenously 
welded (Fig, 9). The landing shock is absorbed by the telescop- 
ing struts. The shock absorbers operate by compression of air, 
sealed and regulated by oil. In this way the kinetic energy of 
the airplane is converted into friction and heat, and the un.- 
avoidable springing in landing with rubber shock absorbers is 
eliminated. With a stroke of about 24 cm (9.44 in,), ant energy 
absorption! of about 45^ of the total weight is attainable. The 

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N.A.C.A. Technical llsraorandum IJo. 523 5 

absorbed energy corresponds to a free fall of about 0.5 ra (1.64 
ft.) of the fully loaded airplane. The unabp-orbed forces tixe 
transmitted to the fuselage. 

Aside from the main structured parts, vrhose joints are al- 
most exclusively welded, the gas-welding process is used to ad- 
vantage in vll kinds of fittings, connections, levers, etc. 
Properly wrelded parts satisfactorily replace parts cut out or 
forged in one piece. The ¥;eldlng process is simpler, quicker, 
and cheaper aid. saves much weight, which is a very important 
consideration in airplane construction. 

For the different materials available for welding in 
craft construction, adaptations of the welding wire have been 
discovered, which render the welds, as improved by a,fter-trea,t- 
ment, very nearly as strong a,s the unwelded tubes. The excel- 
lence of the weld depends largely on the skill of the welder, 
who should have some understanding of the physical process in- 
volved. In training welders, their individual qualifica.tions 
must be considered first of all. The inspectors must be able 
to tell the difference between a good weld and one that simply 
adheres or is burned. 

For testing the welds, portions are taken from the completed 
structures, from which semples are prepared for tensile and 
bending tests. A bending test is made of the finished part, in 
order to determine its buckling strength. This is a very good 
test of the excellence of the welds ajid exposes any defects in 
the welds. 

N.A.C.A. Technical Memorand-um Ho, 533 6 

It is generally assimied that the excellence of a welded 
structure depends on the strength of the v/eld seam, since this 
is ordinarily the weakest point. I c3Jinot accept this assump- 
tion unconditionally, however, since the vield seam may 'oe the 
strongest part of the structure "under certain conditions. I 
will omit the discussion of the vrelding wire to be used, as "be- 
ing too far reaching. I will only mention that it is important 
to test the welding vdre thoroughly for the different uses. 
Autogenous welding has now become very important in a.irplane con- 
struction* It facilitates the construction and shortens the 
time required. 

The new industries of aircraft construction and autogenous 
welding constitute a timely coincidence. In 1903 the first 
flights were made with engine-propelled airplanes, ^oid in., the 
same year the first burners for acetylene-oxygen welding were 
put on the market. 

Although the laws of statics are fundamental for construc- 
tion, progress is made through the properties of the materials 
used, as determined by experience. The type of girder used for 
withstanding bending and buckling stresses in aircraft is al- 
ready regarded with increased respect in other lines of construc- 
tion:. The autogenous welding industry is one of the few indus- 
tries which can increase its usefulness through the discovery 
and appropriation of new fields of applicatioir-. There are many 

such fields not yet aware of the fact that production! can be 
cheapened by the use of autogenous welding. 

N.A.C.A. Technical Memorarxdum No, 533 7 


Mr. Herz .- I only wish to ask whether autogenously welded 
or seamless drai.'m tubes were used: 

Dr. Kuchel .- Seamless dravm steel tubes, as obtainable in 
the m.ELrket, are generally used in airplane construction. There 
is no need of producing them in the aircraft factory, because 
every kind of tube caji be bought ready made in the market. 

Mr. Herz .- My question was meant somewhat differently. In 
recent years quite an extensive industry has developed in the man- 
ufacture of T/ell-made autogenously v/elded tubes for all purposes. 
Have these tubes yet been used in aircraft construction? I vn- 
derstand that such tubes were investigated in Italy in 1923 ox 
1924. I do not know, however, what the outcome was. 

Dr. Kuchel .- I do not know of au.togenously welded tubes 
being used in aircraft construction either in Germany or else- 
where. In a tour of investigation in the spring through France 
and England, I found, however, that this kind of welding is done 
with no such precision in either of these countries as in Germany. 

Mr . But z . - In the ivelding of thin-walled tubes, are there 
any data available on the effect of the purity of the welding 

Dr. Kuchel .- The vrelding section generally uses e,pparatus 
in which the gas is purified in the ordinary way. It is endeav- 
ored to keep the gas as cool as possible, to have it well \7ashed 

N.A.C.A. TechniccJ. Memorpjiciuni ITq. 523 8 

ejid to the purifying r.;c.teri;:J.G renewed often enough. 

llr . But z , - I recently ascertained the effect of the purity 
of the oxygen in a large v/elding factory ivhich produces thin- 
walled tubes rjid "bicycle frames in large quantities. This fac- 
tory undertook to use high percentc^-e oxygen. The rejections 
were 30^ greater thoji with oxygen under 98^ pure. The demcjids 
on the skill of the weldes increase with the purity of the oxy- 
gen, especially for thin-walled tubes. 

Mr. Pothmann .- ¥e found that the purer the gas, the more 
sensitive the flojne wa^s when \7r0ngly adjusted. We also foiJ-iid 
that oxidation occurred much more readily with very pure gas. 
In ordinoa'y welding poorer results were obtained with very pure 
gas {99,5fo). 

Dr. Streb ,- According to our experiments, nitrogen up to 
5fo has no hcj?mful effect on the quoJ.ity of the weld. We have 
not yet determined v;hether a smaller amount of nitrogen has an 
actually favorable effect on the quality of the weld. There 
were some indica.tions in other experiments that the presence of 
a small quantity of nitrogen in the oxygen prevented or substan- 
tially reduced the carbonization of the weld, when a slight ex- 
cess of 3,cetylene was used. 

Dr. Vogel (the presiding officer).- It seems that the ex- 
perimental results do not yet justify their adoption: in practice. 
This remaxk applies also to the results thus far obtained by 
Dr. streb. 

Translation by Dwight M, Miner, 

JTational Advisory Comri'iittee for Aeronautics. 


N.A.C.A, Technical Memorandum No. 533 

FlgB.1,3,3,4,5.6,7,6,9 ' 


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